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T/S distance is the distance between the cathode and the substrate in a sputtering equipment, or more precisely, from the target surface of the cathode to the substrate.
For a fixed cathode, the T/S distance cannot be altered, but certain equipments have an adjustable top and bottom cathode mechanism, or flanges for adjustment. T/S distance affects film formation rate and film thickness uniformity. Appropriate distance is crucial, as the discharge would not occur if the distance is too short or long.
The typical T/S distance for a 4 -6 inch cathode is about 40 -70 mm, though the precise conditions vary on the pressure at the time of discharge. As the T/S distance increases, the film formation rate drops. Since the effect on the film thickness uniformity cannot be generalized, optimal value must be determined based on the material and the substrate size.